A family of standards has been developed by the Institute of Electrical and Electronic Engineers (IEEE) to provide for fixed, portable, and/or mobile broadband wireless access networks (for example, the IEEE std. 802.16e, published 2005).
In some current systems, certain information is typically “signaled” from a base station to a mobile station so that the mobile station can properly encode (in the case of uploads) and decode (in the case of downloads) bursts of data. The signaling method used in current IEEE 802.16e compatible systems is referred to as MCS (modulation and coding scheme).
MCS signaling means that there are a small number of pairs of code-rate (R) and modulation order (M), and the signaling selects one of them. Knowing the allocation size in terms of number of quadrature amplitude modulation (QAM) symbols, the burst size is computed as: NQamSymbols·R·M for the selected pair (R, M).
One problem with MCS signaling is that the set of possible burst sizes depends on allocation size. For example, in IEEE 802.16e, a burst size of 45 bytes is possible only if five slots are allocated. This makes it difficult to avoid padding in bursts which results in wasted bandwidth. For example, for large burst sizes, padding may be avoided by fragmentation or concatenation of physical layer data units (PDUs); however, this requires the media access control (MAC) layer to be aware of momentary scheduling and link adaptation decisions, which make it a difficult problem. Also, for example, for small burst sizes, or latency-limited applications (VoIP, gaming), the PDU size actually comes from higher layers and padding is inevitable.